The present invention relates to a linear image sensor for capturing charges by lines of light receiving pixels and transferring them to acquire an output signal, an image reading device and a charge transfer method.
In a color document reading device such as a color copier and color scanner, three-line color linear sensor provided with three lines of light receiving pixels for reading Red light, Green light and Blue light.
Generally, a three-line color linear sensor for the purpose of high speed reading is provided with signal charge transfer sections on both sides of each of the Red light receiving pixel line, Green light receiving pixel line and Blue light receiving pixel line so that the signal charges at even number pixels and odd number pixels are separated to provide parallel outputs corresponding to two channels for each color and hence a total of six channels, thereby realizing high speed read in some degree.
In recent years, the need for the high quality of read image has increased the density of a required pixel from 400 dpi (dot per inch) to 600 dpi or more. Correspondingly, the unit pixel area at the light receiving section of the image sensor has been reduced.
Thus, the signal charge generated in a photo-diode section at the same light intensity is reduced and hence the substantial sensitivity is also reduced. In order to compensate for such a reduction, an efficiency of converting the signal charge into a voltage at an output section must be improved. However, an increased gain principally has an adverse effect on the frequency characteristic of the output section. This makes it very difficult to realize both high speed and high sensitivity of signal reading.
At present, the scanning speed in a sub-scanning direction which permits high quality color reading at a read pixel density of 600 dpi has an upper limit of 210 mm/sec. The speed of the sub-scanning direction is attributable to the limit of the frequency characteristic of the output amplifier section which reads out the signal of each pixel of the sensor.
Generally, where the outputs from the light receiving pixel lines are transferred by the signal charge transfer sections corresponding to two flows consisting of odd number pixels and even number pixels, the transfer speed at a transfer electrode has a margin, but is limited by the upper limit of about 20 MHz of the frequency characteristic of the output amplifier section. This results in the transfer speed of about 40 MHz at a totaled video rate in the two flows.
Thus, 7500 pixels, which are required to read a document having a A3 size (JIS standard) at a read pixel density of 600 dpi, can be read at the reading speed:
40 MHz÷7500 (pixels)÷23.6 (pixels/mm)=226 (mm/s).
However, in view of the shifting period of the signal charge and transfer period of the ineffective pixels, the reading speed is limited to 210 mm/sec described above.
A digital full color copier which is generally used in an office is provided with a three-line color image sensor capable of making high speed reading described above at the document reading section. In order to copy a good full color image in the digital full copier, the tone of each color signal in the document to be read is important. For example, the reproductivity of 256 tones for each color signal is required.
Therefore, when the document is read, in order to assure a sufficient quantity of exposure light incident on the three-line color image sensor, an illumination optical system or a read scanning speed must be set.
On the other hand, in the digital full color copier having such a setting, where a monochromatic image which has a text document or linear image to be mainly copied is copied, unlike copying of the full color image, the improved tone reproductivity is not valued, but the high speed permitting a large number of sheets to be taken per a unit time is required.
In order to realize compatibility between the high quality full color copying function, which utilizes the function of digital reading by a single copier, and the high speed monochromatic copying function of an analog copier, the constraint for the high quality full color reading becomes a burden on the high speed monochromatic image reading.
Specifically, when the monochromatic image is read using the same image sensor as when the full color image is read, it can be read only at the speed equal to that in reading the color image. Therefore, the monochromatic image cannot be read at the high speed equal to that in an analog copier dedicated to the monochromatic image.
Meanwhile, when plural sheets are copied for a sheet of document, the information for a sheet of document stored in a memory may be continuously transferred to a printer section. This eliminates the burden of the high speed on the reading section.
However, when different documents are continuously copied one by one by an attached document automatic feeder, the same information cannot be produced from the memory. This requires the documents to be read at a high speed.
As a xe2x80x9cmonochromatic sensorxe2x80x9d, a CCD linear image sensor capable of reading the document at a high speed as shown in FIG. 8 has been proposed (JP-A-4-243138). In the CCD linear image sensor, the charges captured by a single line 100 of photosensitive pixels are transferred in a manner divided into two flows of CCD shift registers 101 and 102 for odd and even number pixels. On the way, the charges are transferred in a manner distributed into four flows consisting of two CCD shift registers 101a and 101b distributed from the CCD shift register 101 and two CCD shift registers 102a and 102b divided from the CCD shift register 102. Such a transferring manner permits the high speed of the charge transfer.
Assuming that the upper limit of the frequency characteristic in each output section is e.g. 20 MHz, the above configuration for parallel outputting of the four flows permits the signal charges to be output at the maximum data rate of 80 MHz.
However, if the above configuration is applied to a three-line color image sensor as it is, a total of 12 flows are required to capture the signals for three colors. This leads to an increase in the width of a sensor chip and large-scaling of an output processing circuit such as an analog circuit and an A/D converter.
Also, in the case where only the output section for capturing only the one color in the three-line sensor is designed in four flows, the output sections corresponding to a total of eight flows are required. This gives rise to the same problems as described above.
The linear image sensor according to the present invention comprises: a first and a second light receiving pixel lines for making photo-electric conversion; a first charge transfer means for transferring charges acquired from the first light receiving pixel line through the photo-electric conversion; a second charge transfer means for transferring charges acquired from the second light receiving pixel line through the photo-electric conversion; and a first branch transfer means for branching the charges from the first charge transfer means and transferring them to the second charge transfer means.
The image reading device using a linear image sensor according to the present invention comprises a first and a second light receiving pixel line for making photo-electric conversion; a first charge transfer means for transferring charges acquired from the first light receiving pixel line through the photoelectric conversion; a second charge transfer means for transferring charges acquired from the second light receiving pixel line through the photo-electric conversion; and a first branch transfer means for transferring the charges branched from the first charge transfer means to the second charge transfer means.
In the above linear image sensor and image reading device using it, the first branch transfer means permits a part of the transferred charges to be further transferred using the charge transfer means for the adjacent light receiving pixel line.
The charge transfer method according to the present invention for a linear image sensor including a first and a second light receiving pixel line for making photo-electric conversion comprises the steps of: transferring charges acquired from the first light receiving pixel line through the photo-electric conversion by a first charge transfer means; transferring charges acquired from the second light receiving pixel line through the photo-electric conversion by a second charge transfer means; and branching the charges from the first charge transfer means at a branching point and transferring them to the second charge transfer means.
In the charge transfer method according to the present invention, the charges can be transferred at a high speed before the branching point and they can be transferred after the branching point.